Today it means: An isolation exercise for the quadriceps femoris in which, from a standing position, the knees bend forward while the hips remain extended and the lifter leans backwards. The heels normally lift off the ground and the tibia reaches a mostly perpendicular angle with the direction of resistance(gravity). The knees do not need to touch the floor before returning to a standing position. Optionally, weight can be held on the torso in order to increase the load on the quadriceps femoris. Full definition →
The dependent “sissy” is a cranberry morpheme in reference to the mythological figure of Sisyphus here, owing to the exercise isolating all heads of the quadriceps femoris, while performing step-up-like exercise similar to that of Sisyphus also entails having large quads.
Etymology from Wiktionary, CC BY-SA 4.0.